Impact crater ejecta is typically brighter than the surrounding material because it is fine-grained and immature (unweathered); even on the dark mare, fresh craters usually have bright ejecta. Craters with distinctly dark ejecta do occur, but they are rare (e.g., Shorty Crater at the Apollo 17 landing site). When craters have dark ejecta, the interpretation is that a layer of low reflectance rock or soil at depth was excavated and distributed around the margin of the crater. In the case of Shorty Crater, the conclusion is that a layer of dark pyroclastics was hidden just beneath the surface; in other cases, dark halo craters are interpreted to indicate mare material at depth (cryptomare). Several of the craters formed by the impact of spacecraft hardware, such as the Apollo 13 S-IVB, into the surface also have dark ejecta rays, and this observation is not yet well understood.

Details of the crater morphology can be seen in this expanded view from LROC NAC M1115555142L [NASA/GSFC/Arizona State University].

Our dark halo crater (31.131°S, 147.536°E) has a diameter of about 78 m, although it is slightly elongate in the north-south direction. Rays of dark ejecta extend for almost 200 m from the crater rim. Morphologically, the crater is not the normal simple bowl shape for a crater of this size. Rather, a depression on the crater floor in the center is surrounded by a low ridge about 33 m in diameter; beyond that annular ridge to the crater wall the floor appears to be flat. Boulders are scattered on the crater floor and on the ejecta to the east; the largest boulders on the east side are as large as 6 m across.

Regional view of the dark ejecta crater. Note the crater formed on the western flank of an older, larger crater. Relatively low albedo, smooth plains spread out immediately to the south. These plains (see next image) may be mare material and may underlie the area of the small impact. LRO NAC frame M167241339R (spacecraft orbit 9780, August 6, 2011; 55.96° angle of incidence, 65 cm per pixel resolution from 62.95 km) [NASA/GSFC/Arizona State University].

LROC Wide Angle Camera (GLD100) context, at 64 meters per pixel resolution, shows the location of the crater of interest in relation to the arc of exposed mare material to the south and west [NASA/GSFC/Arizona State University].

This crater formed on the outer flank of a larger older, degraded 635 m crater. Highlands ejecta (higher reflectance than mare) from this larger crater buried the mare. Later the impact that formed the younger dark halo crater punched through the bright highland ejecta and brought up mare from 10 meters or more depth.

The Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft is currently circling the Moon. With the spacecraft safely settled into its observation orbit, the mission science team is busy testing and calibrating its instruments. This U.S. mission was designed to characterize the lunar “atmosphere” – the extremely tenuous zone of gases that vary in time in the space above the Moon. Technically called an exosphere, this region contains extremely low concentrations of a variety of elements and compounds, of varied origins and a largely unknown life cycle. LADEE is designed to monitor and characterize these species, with the goal of identifying the process and sources of the gases and how they vary with time.

Initially a precursor to human lunar return, LADEE was selected for development early in 2008, as we wanted to understand the lunar exosphere before the lunar environment was contaminated by humans. The LADEE spacecraft is designed to observe the Moon in its natural, pristine state. However, the very act of going to the Moon inadvertently (though briefly) modifies the lunar atmosphere. When a spacecraft arrives at the Moon, it uses its on-board rocket engines to brake into lunar orbit or to descend to the surface. These rockets spew large quantities of exhaust gas into space and as the vehicles get captured into the Moon’s gravity field, so too does this exhaust product.

From estimates drawn on the Apollo landings, the rocket exhaust expended from each Lunar Module temporarily doubled the total mass of the natural lunar atmosphere. This artificial addition of gases eventually dissipates, driven off by solar interactions and other complex effects. In time, the Moon resumes its normal state of near-vacuum. The creation of a temporary artificial atmosphere created by rocket effluent and its subsequent dissipation is imperfectly understood, except to the extent that we know that it happens. The one-month “commissioning phase” that the LADEE mission is currently experiencing was largely designed to ensure that the exhaust from the orbital braking burn of the spacecraft (and subsequent low-rate out-gassing from the spacecraft) is largely complete. We want to measure the Moon’s environment, not the products of the craft that brought us there.

But the U.S. will not be the only one conducting a mission at the Moon for the next few months. The long-planned Chinese robotic mission Chang’E 3 is scheduled for launch to the Moon in early December. Their lander mission will place a fairly large (1200 kg) spacecraft on Sinus Iridum in the northwestern quadrant of the near side, deliver a small roving vehicle and examine and measure the properties of the lunar surface over the course of several months. But before it begins its surface mission, the Chang’E 3 spacecraft will burn roughly 2600 kg of rocket fuel in the vicinity of the Moon’s exosphere. I have not seen any documentation on the fuel this spacecraft will use, but it is highly likely that it will be the chemicals unsymmetrical dimethylhydrazine (UDMH; H2NN(CH3)2) and nitrogen tetroxide (N2O4). These propellants are widely used in spacecraft because they are liquid at room temperature and can be easily stored in tanks for long periods of time (a requirement for long-duration spaceflight to destinations beyond low Earth orbit).

When UDMH and nitrogen tetroxide are burned in a rocket engine, they produce a variety of exhaust gases; the dominant combustion products are water (H2O), nitrogen (N2), carbon monoxide (CO), carbon dioxide (CO2), and a few trace species, including hydrogen (H2) and hydroxyl (OH). Expelled by a rocket nozzle, these gases rapidly expand in all directions in the vacuum of space. Because most of the burn occurs after the spacecraft has been “captured” by the gravity of the Moon, this rocket exhaust is also captured by the Moon. Thus, exhaust from an orbital or a landing vehicle becomes (temporarily) part of the lunar atmosphere.

If you’re thinking that this “rude” addition of alien gases will mess up the very delicate phenomena that LADEE was designed to map and measure, you’re correct – it does. You might even expect the scientists of the LADEE team would be very upset at this disruption of their carefully planned measurement strategy. But you would be wrong. This problem is actually an opportunity.

If successful, Chang'e 3 will perform the first soft-landing on theMoon since Luna 24 in1976 and deploy the first lunar rover since Lunokhod 2 in 1973 [NASA/CNSA].

The coincidence of Chang’E 3 arriving at the Moon after LADEE has begun observations has developed into a serendipitous occurrence for lunar science. Because we don’t understand very well how exospheric gases are added to and removed from the Moon, what has landed in our laps is an unplanned (but controlled) experiment. A known quantity of gases – of known composition – will be added to the lunar atmosphere at a precisely known time, in a precisely known place. One could have not designed a better experiment to measure how this addition of material is distributed, how its distribution evolves over time, and how these expelled gases dissipate into cislunar space. Even better, LADEE will have almost a full month to monitor and characterize the lunar atmosphere before Chang’E arrives, thus allowing us to first observe the “natural” Moon and then the “contaminated” Moon and how the lunar atmosphere recovers from its defilement.

None of this was prearranged – the Chinese schedule their missions on the basis of their own time-table and programmatic needs (just as NASA’s lunar goals have changed over the last 5 years). But because of a fortuitous alignment of schedules, we have a unique opportunity to observe in real time how the Moon works. Hopefully, the Chinese will provide us with detailed mass numbers of their spacecraft and exactly what variety of fuel it carries, but even if they don’t, physics dictates a certain mass and volume of the exhaust gas and its composition will be measured by LADEE (allowing us to know the type of fuel used). China’s December lander mission to the Moon will provide our U.S. mission with a welcome bit of “traffic exhaust,” giving scientists the opportunity to learn more from LADEE than we’d originally envisioned.

Tuesday, October 29, 2013

Animations, even notional illustrations, of the up-coming CNSAChang'E 3 lunar lander and its rover have been nearly non-existent. Those of us who have learned fresh pictures are essential for our readers to pay attention to even the most crackling fresh textual news have had to use and re-use often hazy pictures, some from conferences nearly a decade old.

Hopefully, that's beginning to change, perhaps as much a result of labors of love and a broader availability of modeling software beyond any change of priorities on the part of CNSA. That agency's web presence, without the regular help of China's state new agency Xinhua, is plainly useless. Unfortunately Xinhua's reports, though arriving now with increased regularity, are rarely accompanied with any illustration.

But, let's face the sad fact. NASA, JPL, and Goddard's Science Visualization Studio (SVS), as well as very talented subcontractors like John Frassanito & Associates have spoiled us.

Of course, George Lucas' Industrial Light and Magic and Hollywood's regular use of CGI played an even bigger role in advancing our expectations. We have come to expect mission-oriented stills and animations, even real time simulations, as essential to news of the most obscure flights. It meets a demand where the rule of media has long been "no picture, no story."

Still from unprecedented, professional class animation celebrating the upcoming Chang'e 3 mission to the lunar surface [Pockn].

Fortunately for us, someone about whom we presently know very little, beyond a Weibo blog we are still translating and a YouTube account profile ("Pockn Liu"), has just published a brief but outstanding animation celebrating Chang'e 3, about to become the third of China's unmanned lunar missions, now being readied for a December launch.

If successful, when Chang'e 3 lands, as planned, in Sinus Iridum, China will accomplish the first soft-landing on the Moon in nearly four decades, since the Soviet Union's Luna 24 in 1976.

Friday, October 25, 2013

NASA's Lunar Laser Communication Demonstration (LLCD), onboard the LADEE lunar orbiter, has made history using a pulsed laser beam to transmit data over the 400,000 km between the Moon and Earth at a record-breaking download rate of 622 megabits per second (Mbps).

LLCD is NASA's first system for two-way communication using a laser instead of radio waves. It also has demonstrated an error-free data upload rate of 20 Mbps transmitted from the primary ground station at White Sands, New Mexico to the spacecraft orbiting the moon.

"LLCD is the first step on our roadmap toward building the next generation of space communication capability," said Badri Younes, NASA's deputy associate administrator for space communications and navigation (SCaN) in Washington. "We are encouraged by the results of the demonstration to this point, and we are confident we are on the right path to introduce this new capability into operational service soon."

Since NASA first ventured into space, it has relied on radio frequency (RF) communication. However, RF is reaching its limit as demand for data capacity continues to increase. The development and deployment of laser communications will enable NASA to extend communication capabilities such as increased image resolution and 3-D video transmission from deep space.

"The goal of LLCD is to validate and build confidence in this technology so that future missions will consider using it," said Don Cornwell, LLCD manager at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "This unique ability developed by the Massachusetts Institute of Technology's Lincoln Laboratory has incredible application possibilities."

LLCD is a short-duration experiment and the precursor to NASA's long-duration demonstration, the Laser Communications Relay Demonstration (LCRD). LCRD is a part of the agency's Technology Demonstration Missions Program, which is working to develop crosscutting technology capable of operating in the rigors of space. It is scheduled to launch in 2017.

Today's Featured Image highlights the northern portion of the ejecta deposit splashed from an unnamed crater (~2.7 km in diameter, about 5 km south of this image) located in the highlands ~800 km northwest of Mare Orientale. The curved stripes from the bottom toward the upper-left of this image represent the flow direction of ground hugging ejecta deposits.

As shown in the classic impact cratering model, the ejecta is ballistically sprayed out of the impact center forming an ejecta curtain. After the ejecta curtain impacts the outside of the crater, it flows in a ground hugging horizontal motion until its kinetic energy is completely dissipated. The ejecta direction depends on flow speed and the local topographic slopes or undulations.

Nearly the same field of view with the Sun overhead results in an image that puts the emphasis on albedo over topography (see the wide context in the image below), LROC NAC M18660602LR, LRO orbit 12574, March 17, 2012; angle of incidence 8.05° at 1 meter per pixel resolution, from 120 km [NASA/GSFC/Arizona State University].

8280 meter-wide field of view from LROC NAC M186606026LR puts the highlighted topographical low in context with the source of the ejecta blanket from the fresh crater 5 km south [NASA/GSFC/Arizona State University].

Further context on the unnamed crater and surrounding ejecta from a LROC Wide Angle Camera (WAC) monochrome mosaic (100 meters per pixel) centered on 9.34°N, 250.08°E. The LROC NAC M130008764R footprint and location of the field of view shown at high resolution in the LROC Featured Image are designated [NASA/GSFC/Arizona State University].

As seen in this second image, the flow direction of ejecta curved along a bowl shaped topographic low (probably a degraded old crater). These characteristic flow lines following the local topography allows scientists to estimate the actual flow speed. In turn, these estimates elucidate detailed mechanisms of ejecta emplacement on the Moon and by comparison other airless bodies, such as asteroids and the planet Mercury

Much wider context from LROC WAC (GLD100) mosaic puts the are of interest well within the secondary bombardment ejecta originating from the basin forming impact that created Mare Orientale [NASA/GSFC/Arizona State University].

Tuesday, October 22, 2013

Close-up on fractured impact melt ponding high on the west wall of Jackson crater. LROC LROC Narrow Angle Camera (NAC) M182253065R, LRO orbit 11965, January 26, 2012. Image 2130 meter-wide field of view centered on 22.534°N, 195.59°E, incidence angle is 57° at 1.47 meters per pixel resolution in the original [NASA/GSFC/Arizona State University].

Hiroyuki Sato
LROC News System

The opening image highlights a fractured pond of impact melt rock inside Jackson crater (72 km diameter). This prominent farside crater is known by its prominent ray system and large amount of impact deposits. Melt pooled not only at the bottom of the crater floor, but also on terraces of the interior wall.

The fractured melt sheet in the opening image is found amongst a grouping of melt lakes on a western crater wall terrace (see WAC context image below).

Western part of Jackson crater and surrounding areas in LROC WAC monochrome mosaic (100 m/pix). The NAC footprint (blue box) and the location of opening image (yellow arrow) are indicated [NASA/GSFC/Arizona State University].

Detached fragments from the main body of melt sheet look like a jigsaw puzzle (upper smooth surfaced portion of the image). These fragments give an impression of the thin and brittle nature of solidified impact melt. Shadow lengths show these fractured pieces to be 5 to 8 meters thick. What caused the once level and smooth ponded surface to fracture? We don't know for sure, but by looking at the whole area a plausible story can be imagined. The south part of the melt lake with the fractured plates is connected to another melt lake at a lower elevation. Perhaps drainage of subsurface unsolidified melt might have dragged a crust toward the south and broken the it into many blocky pieces. Tectonic deformation of the crater wall, perhaps consisting of whole terraces deforming, also might have occurred which could deform the brittle crust of melt ponds.

Launched last month from the Wallops Island site, LADEE (for Lunar Atmosphere and Dust Environment Explorer) will spend the next few months orbiting the Moon. This small spacecraft will attempt to characterize and measure the lunar “atmosphere,” while also looking for dust that might be electrostatically levitated above the surface or thrown into ballistic flight by impacts.

Wait a minute. Did I say “atmosphere?” Isn’t the Moon renowned for its lack of an atmosphere? Indeed it is. In fact, the 10-12 torr surface pressure of the Moon is a better vacuum than we can achieve with even the most advanced equipment in Earth laboratories. (For comparison, sea level pressure on the Earth is about 760 torr, making the lunar surface pressure over one hundred trillion times less dense.) A better term for the tenuous gas near the Moon is “exosphere,” meaning free flying gas molecules that may or may not be gravitationally bound to the Moon. In such an “atmosphere,” there may be only a few thousand molecules in a cubic centimeter of space. This is very tenuous indeed.

After the Commissioning phase of its mission is complete, the spacecraft's current 250 km circular orbit will be reduced further down to within 50 km to begin its 100 day Science Mission [NASA/GSFC].

LADEE is designed to investigate from where these atoms and molecules come. Presently, we think the lunar exosphere consists mostly of helium, sodium and perhaps argon atoms, each coming from a completely different source. Helium likely comes from the Sun, as the solar wind continually “breathes” onto the surface of the Moon. Some atoms stick to surface dust grains but many simply bounce off, randomly moving in the space above the lunar surface. Easy to detect, lunar sodium has been observed from Earth-based telescopes. It most likely comes from rocks vaporized by the continual rain of micrometeorites. At least some fraction of this vaporous sodium must hang around the surface, unable to escape the Moon. Argon might have a solar wind origin, but at least some of it comes from the natural decay of radioactive potassium in the lunar interior (potassium-40 (40K) decays to argon-40 (40Ar) with a half-life of a bit more than one billion years). Gases like argon, venting from the interior of the Moon, were observed by subsatellites left in lunar orbit by the departing Apollo spacecraft over 40 years ago (these small spacecraft have long since crashed into the Moon).

Although helium, sodium and argon are the principal expected components of the lunar exosphere, the LADEE team will search for other species. An interesting possibility is water (H2O) or its related species, hydroxyl (OH). One of the most surprising results of recent lunar exploration was the discovery of adsorbed (surface) water and hydroxyl on the dust grains of the lunar surface (observed by the Moon Mineralogy Mapper (M3) aboard the Indian Chandrayaan-1 lunar orbiter in 2009). Occurring in the form of a monolayer of molecules on dust grains in the cooler portions of the Moon, a clear water signal is best seen above latitudes of 65° and increasing in strength (i.e., increasing water abundance) toward each pole.

The surprise from M3 was not only the presence of water but observing that its abundance increases with decreasing surface temperatures. This means that water being made or deposited on the surface is in motion, with a net movement toward the poles. Chandrayaan-1 also carried an impact probe with a mass spectrometer. During the probe’s half-hour descent to the South Pole, it passed through a cloud of water in space, just above the lunar surface. The water cloud at this high latitude had a density a hundred times higher than at the equator, providing additional evidence that exospheric water is in motion, moving from lower, hotter latitudes towards higher, cooler ones.

LADEE cannot directly measure this water in a neutral state, but if some process ionizes it (e.g., if a water molecule breaks apart into a proton and a hydroxyl by UV radiation from the Sun), it will be visible to the ultraviolet spectrometer aboard the spacecraft. If the process of water migration on the lunar surface is correct, we should be able to observe exospheric water and by measuring its density with time, track the water migration to higher latitudes.

Lunar Horizon Glow (LHC) observed for several hours following local sunset from Surveyor 7 and its landing site just north of Tycho crater. [NASA].

LADEE will also tackle another controversial issue – the amounts and mechanisms of dust movement on and around the Moon. During the unmanned Surveyor lander missions over 40 years ago, a strange illumination or glow was observed by television for several hours after local sunset, just above the horizon. This phenomenon was termed “horizon glow” by surprised Surveyor investigators. At a loss to explain it, the team postulated that some mechanism was lofting dust up above the surface and this dust was scattering sunlight. Exactly how the dust was lofted was uncertain; some thought it must be fragments in ballistic flight from distant impacts, while others thought that it might be levitated by electrostatic force, thus “hovering” above the surface.

A few years later, just before his orbiting spacecraft emerged into the daylight side of the Moon, Apollo 17 Commander Gene Cernan observed and sketched an illuminated limb and “streamers” that could be seen extending into space above where the lunar horizon would be. At the time, this phenomenon was thought to be the same as that seen in the Surveyor pictures, although they have totally different scales (the Surveyor horizon glow must occur within a few meters of the surface, while Cernan’s horizon glow extended many kilometers above the Moon). Dust (probably of lunar provenance) is certainly involved in whatever causes this horizon glow.

As the Moon slowly rotates once every 708 hours, the line between the sunlit and dark hemispheres (the terminator) slowly moves across the lunar surface. The day and night hemispheres have different fluxes of electrons from the solar wind and thus, the presence of the terminator can induce an electrical charge in surface materials. It is postulated that this charge might levitate smaller dust particles such that they would hover above the surface. LADEE will attempt to detect and map this dust, both by searching for scattered sunlight with its ultraviolet spectrometer and via the direct detection of dust particles in flight with an instrument on the top of the orbiting spacecraft.

The issue of levitated dust is thought to be relevant to the future habitation of the Moon. If dust is lofted above the surface by the passage of the terminator, the particles could degrade clean surfaces and create a hazard for inhabitants of the Moon. Such a process could have major effects near the poles of the Moon, areas that are in the near-constant presence of a day-night terminator. Although it is unlikely that levitated dust on the Moon is an environmental hazard, we currently are working in near total absence of hard data. Thus, it makes sense to at least try to make some direct measurements of the dust environment around the Moon to assess the importance of this proposed surface process.

LADEE arrived in lunar orbit last Sunday. We wish it well on its mission to give us fresh (and welcome) data on a poorly understood aspect of lunar processes and history.

Tuesday, October 15, 2013

Synthetic view of the waxing Moon, created using a ray trance algorithm and data collected since 2009 by the enduring lunar orbiter LRO, including over 110,000 LROC Wide Angle Camera (WAC) observations. Simulated perspective from Earth, October 15, 2013, 17:00:00 UT [NASA/GSFC/Arizona State University].

Raquel Nuno
LROC News System

Today’s Featured Image is a simulated view of the Moon as seen from the Earth on Tuesday 15 October 2013 at 10 AM Mountain Standard Time (1700 UT, the time this blog post was released).

The Moon will not be in the sky over North America, but this is what the Moon will look like over Mumbai, India, where the local time in Tempe, Arizona will be 10:30 AM. This synthetic view was generated using a ray-tracing algorithm to place shadows and shading into the LROC WAC normalized reflectance map based on the topography portrayed by the WAC Global Lunar DTM (Digital Terrain Model) 100 m topographic model, or GLD100, and the LRO Lunar Orbiter Laser Altimeter (LOLA) topographic model of the lunar poles.

The normalized WAC reflectance map was generated from 110,000 WAC images, removing the effects of varying lighting geometries on the reflectance by calculating (or normalizing) reflectance at common illumination and viewing angles everywhere on the Moon (at the resolution of a 100 meter grid). For a detailed explanation of how the normalized map was generated check out this Featured Image post (and this spectacular rotation movie (100 plus mg .mov file) of the same map).

The second data product used to generate today’s synthetic view of the Moon was a global topographic map, that is a combination of two digital elevation models: the GLD100, and the LOLA topographic model of the lunar poles. We then combine the topography and the normalized images by draping the normalized reflectance map over the global topographic map; this step places the reflectance pixels in 3D space (latitude, longitude, radius). At this point the location and orientation of each pixel is known relative to the Sun such that shadows and shading can be accurately added to the reflectance map. A ray-tracing algorithm steps back from each pixel to the location of the Sun, and if it encounters another pixel along the way, the pixel is shadowed (see image below).

Finally, the relative reflectance is computed for each unshadowed pixel based on the calculated illumination and viewing angles. In a nutshell, we can generate a new view of the Moon at any lighting condition. Since the location of the Sun relative to the Moon is known through time, we can generate an image of how the Moon looked, or will look, on any date, like today's Featured Image!

Visual representation of how the ray tracing algorithm determines if a pixel is or is not in shadow. [Hanger, et al., 2013].

So why bother making synthetic images of the Moon when we can take images? We can see what the lighting will be for a certain feature along LRO’s flight path, and the ideal lighting conditions can be chosen to observe it, allowing us to optimize NAC targeting.

LADEE, last of the Constellation precursor missions, officially entered a highly eccentric orbit around the Moon on October 6, after a month gradually increasing its orbital apogee around Earth to meet up with the Moon's gravimetric Sphere of Influence. Following critical maneuvers October 9 and 12, LADEE is now inserted into a low circular lunar orbit below 250 km to begin the Commissioning phase of its mission, still shy of a much lower Nominal mission orbital altitude below 50 km. [NASA/ARC].

Friday, October 11, 2013

Scott Carpenter, the second American to orbit the Earth and one of the last two surviving Project Mercury astronauts, has died.

His wife, Patty Barrett, said Mr Carpenter, 88, died in a Denver hospice of complications from a stroke he suffered in September.

As an astronaut and aquanaut who lived underwater for the US Navy, he was the first man to explore both the depths of the ocean and the heights of space.

It was Mr Carpenter who gave the famous send-off - "Godspeed, John Glenn" - when John Glenn became the first American in orbit in February 1962. Mr Glenn, 92, is the only member of the "Mercury 7" still alive.

Lunar mare present many excellent examples of wrinkle ridges, where tectonic activity caused the foreshortening of near-surface rocks. The loading of large basins by dense mare basalts is thought to have resulted in isostatic adjustment of the underlying anorthositic crust, leading to buckling and overriding of surface rock units one atop another as compression occurred. The same stresses may also produce extensional (rather than compressional) deformation in adjacent areas. A variety of complex landforms can thus result.

Area shown at high resolution in the LROC Featured Image is designated with a small arrow in this 34.4 km-wide field of view from LROC Wide Angle Camera (WAC) monochrome (604 nm)observation M146911901CE, LRO orbit 6784, December 13, 2010; early morning 78.67° angle of incidence, resolution 59.3 meters per pixel from 43 km. The area of interest is in south central Mare Frigoris. [NASA/GSFC/Arizona State University].

Most of the ridges we see in today's Featured Image are produced by thrust faulting in Mare Frigoris. Just below center in the Featured Image frame, however, is a left-lateral strike-slip fault (also called a sinistral fault). Wrinkle ridges can be lumpy and ropey-looking, not exactly what comes to mind when one thinks of a textbook compressional fault. But in cross section the faulting would be readily apparent (refer again to the links above). Just south of the strike-slip fault are zones of extension where tension cracks have formed (small white arrow in Featured Image).

Further context for the wrinkle ridge in this field of view 107 km wide south central Mare Frigoris, scared by secondary crater streams from Aristoteles crater to the southeast. LROC WAC monochrome mosaic (604 nm) from five sequential orbits captured under local sunrise (emphasizing topography over albedo), LRO orbits 6782-6786, December 14, 2010 averaging a 77° angle of incidence from 43 km [NASA/GSFC/Arizona State University].

Morphologic nuances can be explored elsewhere in the NAC frame. Note the ropey appearance of some of these ridges, again showing that motions within the rock were complex indeed. Other examples of strike-slip faults have been found in association with lobate scarps on the Moon. Recent evidence suggests that shrinkage of the Moon from deeply seated internal cooling may have contributed to the occurrence of some lobate scarps and wrinkle ridges.

More examples of wrinkle ridges from NAC frame M181102837R [NASA/GSFC/Arizona State University].

Secondary craters are produced by debris lofted during excavation of primary impacts. They can be difficult to distinguish from primary impact craters if they are circular with well-developed ejecta patterns. That ambiguity is a concern to scientists when counting craters to determine the ages of planetary surfaces. This age dating approach assumes that impact flux has held steady throughout geologic time following the Late Heavy Bombardment period ending approximately 3.8 billion years ago. The model thus assumes that older surfaces present more craters than younger surfaces, and that one can determine relative ages by counting the number of impact craters per unit area.

A wide field of view shows a wide variety of secondary crater cluster, from near and far, nearby. LROC NAC M1104980770R; about 2.14 km across [NASA/GSFC/Arizona State University].

Crater counts give reliable relative ages, but can also be translated to absolute ages using the radiometric age dates of rocks returned by the Apollo and Luna missions. Obviously counting statistics are skewed if secondary craters are included in the count; these "extra craters" create the false impression of an older surface.

Swarms of secondaries abound. LROC Wide Angle Camera context for the LROC Featured Image, highlighting the area designated with the arrow, on the rim of Tereshkova U, in this 53.5 km field of view, the full width of LROC WAC monochrome (604 nm) observation M167267012CE, LRO orbit 9784, August 8, 2011; spacecraft and camera slewed 8.68° off nadir, angle of incidence 56.66° at 67.5 meters per pixel resolution from 50.59 km over 31.09°N, 142.86°E [NASA/GSFC/Arizona State University].

Often, as in today's Featured Image, the distinction between primary and secondary craters is unambiguous. Here we see a large number of impact craters in a tight cluster. Particularly apparent when exploring the full NAC frame, we can see a clear association within this cluster, not merely a coincidental grouping of separate features spaced in time across many eons. Their ejecta exhibit higher reflectance than most of the craters in the surrounding areas, and they form a tighter grouping than other fresh-looking craters seen elsewhere in the frame.

At 500 meters per pixel, there's no let up in the secondaries peppering the relatively young Moscoviense basin. The cross at center further designates the location, on the rim of Tereshkova U, shown at high-resolution in the LROC Featured Image. LROC WAC mosaic (GLD100) through the LROC Quick Map feature [NASA/GSFC/Arizona State University].

Friday, October 4, 2013

Boulders scattered across mounds on the floor of Inghirami C may be ejecta nested in melt that originated with the basin-forming-impact that created Mare Orientalis and it's extensive basin 3.1 billion years ago. LROC Narrow Angle Camera (NAC) observation M1114645692L, LRO orbit 16501, February 4, 2013, full 64 cm per pixel resolution crop centered at 44.167°S, 285.309°E. LROC Featured Image field of view roughly 500 meters across, 54.74° angle of incidence [NASA/GSFC/Arizona State University].

H. Meyer
LROC News System

The floor of Inghirami C is littered with boulders that are commonly perched atop closely spaced mounds, some of which are as large as 700 meters by 1000 meters. The presence of these mounds and boulders gives the floor of the crater a lumpy appearance. If simple craters are supposed to be bowl-shaped, where did the material come from to form the mounds? Inghirami C is located southeast of Orientale basin and is adorned with spectacular patterns of ejecta during the basin forming event. Could the lumps and boulders be material from Orientale?

Full 3.2 km-wide field of view of LROC NAC M11144569LE, with the area highlighted at high resolution above outlined in yellow. While amorphous mounds and melt mixtures are common features covering floors of larger craters, like Copernican age Tycho, these features appear more cohesive with no intermediate stages of mixture with their surroundings and mass wasting characteristic of far flung boulders [NASA/GSFC/Arizona State University].

A medium resolution, low illumination angle view of Inghirami C shows the hint of an ejecta blanket from the 19 km crater and its blocky floor, in context with immediate surroundings deeply grooved by forces radiant from the Orientalis basin. Chang'e-2 global photographic [CNSA/CLEP].

In the Wide Angle Camera (WAC) context image (below), the walls of Inghirami C are intact. There are no breaches in the walls to suggest that ejecta from Orientale flowed directly into the crater. If you look closely, you can see a hint of an ejecta blanket outside the rim of Inghirami C. The presence of this ejecta blanket (from Inghirami C) and the lack of breaches suggest that Inghirami C formed on top of the Orientale ejecta.

LROC GLD100 mosaic showing Inghirami C nested in the midst of a wide area of terrain grooved and infilled by eject from the Orientalis basin-forming-impact. The same influence, radiant from Orientalis, continues far to the southwest and may explain the atypical fill in and around other craters in the Schickard crater group at even greater distances, including Wargentin [NASA/GSFC/Arizona State University].

Using basic stratigraphic principles, this means Inghirami C formed after the Orientale ejecta formed the ridged terrain (seen just outside the rim of Inghirami C in the context image). If the material in the floor didn't come from Orientale or any other large crater nearby, then it must be native to Inghirami C. It turns out that not all simple craters are bowl-shaped with smooth floors -- and in fact many display textured floors covered in combinations of breccias, impact melt, and ejected blocks that can form mounds. Over time, the mounds erode, leaving the boulders seen in today's Featured Image.

Back to the question: Could the lumps and boulders be material from Orientale? Not directly as established above, but perhaps the irregular forms are due to an impact into a chaotic, unconsolidated massive ejecta flow from the Orientale event! The lunar surface is incredibly complex - everywhere you look there is something new, waiting to be explored!